The operational benefits of an ability to takeoff and land vertically are self evident. Conventional aircraft must operate from a relatively small number of airports or airbases with long paved runways. For commercial transportation, the airport is rarely where you actually wish to go, and is usually crowded causing delays in the air and on the ground. For military aircraft the airbase is vulnerable to attack, and the time expended crusing to and from the in-the-rear airbase increases the necessary range of the aircraft increases the response time.
The first type of heavier-than-air aircraft to takeoff and land vertically was the helicopter. While the helicopter rapidly proved its worth for rescue operations and short range point-to-point transportation, its inherent speed and range limitations have restricted its applications.
In the area of propeller-powered aircraft, the tilt-rotor concept, as tested in the Bell XV-15, seems to offer the best compromise between helicopter-like vertical flight and efficient wing-borne cruise. The tilt-rotor concept is the basis of the V-22 Osprey which is currently under development.
To date, there have only been a few operational jet VTOL designs, the British Harrier and the Russian YAK-36, both of which are subsonic aircraft. While at least one supersonic VTOL design has flown, the Mach 2 Mirage III-V back in 1966, there has yet to be an operational supersonic VTOL aircraft.
This is largely due to the need for increased internal volume for the vertical lift apparatus and fuel capacity. Also, most concepts for vertical lift tend to increase the aircraft's cross-sectional area near the aircraft's center of gravity, increasing the supersonic wave drag. Finally, the state of the art in engine thrust-to-weight ratio has imposed an excessive weight penalty on VTOL designs. It has simply been impossible, up to now, to provide an operational aircraft having both vertical and supersonic forward flight with any practical range.
Like most technical subjects, the area of vertical flight has spawned its own arcane terminology. VTOL refers to a capability for Vertical TakeOff and Landing, as opposed to Conventional TakeOff and Landing (CTOL). VTOL is the broadest terminology and can be applied equally well to a helicopter or an Apollo space capableaircraft which has the flexibility to perform either vertical or short takeoffs and landings is said to have VSTOL capability (Vertical or Short TakeOff and Landing). An aircraft which has insufficient lift for vertical flight at takeoff weight but which can land vertically at a reduced weight is called a STOVL (Short TakeOff and Vertical Land).
There are a number of unique problems associated with the design and operation of jet VTOL aircraft. Two fundamental problems stand out because they tend to have the greatest impact upon the selection of a VTOL propulsion concept and upon the design and sizing of the aircraft. These are the problems of balance and of thrust matching.
Modern supersonic jet fighters have a thrust-to-weight ratio exceeding one, so it would seem fairly easy to point the jet exhaust downward and, therefore, attain vertical flight at "no cost". Unfortunately, this is complicated by the balance problem. Many subsonic jets and virtually all supersonic jets are designed with the engine at the rear, the cockpit and avionics at the nose and the payload and fuel near the center of the aircraft. This traditional layout places the expendables at the center of gravity, co-locates the parts of the aircraft requiring cooling (crew and avionics), and keeps the avionics away from the hot and vibrating engine.
If the thrust exceeds the weight, vertical flight could be obtained simply by deflecting the thrust downwards. However, some vertical upward force is required forward of the aircraft's center of gravity. In order to balance the vertical thrust force at the tail. This balance problem is possibly the single most important problem encountered in the design of a VTOL jet fighter.
There are two conceptual approaches to solving the balance problem. Either the thrust location must be moved to the center of gravity, or an additional thrust force has to be applied near the nose. Both of these approaches will tend to move the design of the aircraft away from the optimal layout. Generally, jet VTOL concepts can be divided into those which utilize fairly conventional engines and those which use modified engines so that the fan and core air are split, with the fan air ducted and exhausted from some location separate from the core air.
The conventional engine VTOL concepts which do not use additional lift engines for vertical flight must have a net takeoff thrust-to-weight ratio in excess of one. If the jet exhaust is not diverted to some other location for vertical flight, the aircraft must either be a tail sitter (VATOL), or it must have the engine exhaust located at the aircraft center of gravity and capable of vectoring downward for vertical flight. This can be accomplished by using a vectoring nozzle or nacelles which tilt.
The X-14 research aircraft had vectoring nozzles at the center of gravity, with the engines out in front. This is probably not a good arrangement for most applications because the cockpit winds up in the rear for balance, which does not provide acceptable visibility for the pilot. Also, in forward flight, the jet exhaust scrubs alongside the fuselage which causes thermal and acoustic problems.
Tilt nacelles are heavy, but may be the best compromise for some applications. Grumman Aircraft Corporation has been pursuing a tilt-nacelle concept for Naval applications for a number of years.
Some VTOL concepts provide a means of diverting the exhaust flow to provide vertical lift. This is generally done by a retracting blocker device in the engine which shuts off the flow through the rearward-facing nozzle. The flow is then diverted forward through internal ducting. All of these VSTOL approaches, however, exact significant penalties in weight, cost, and completely when compared to a conventional jet aircraft design.
Therefore, it is a primary object of the subject invention to provide VSTOL or VTOL aircraft having a minimum increase in size and weight when compared to a conventional supersonic aircraft designed for a similar mission.
It is another object of the subject invention to provide a VSTOL or VTOL aircraft having a propulsion system located in the aft portion of the aircraft as in a conventional aircraft.
A further object of the subject invention is to provide a VSTOL or STOL aircraft, wherein the exhaust nozzles for both vertical and horizontal flight are located at the center of gravity of the aircraft.